The present invention concerns a method of, and apparatus for, generating pulse width modulation (PWM) signals. More especially the invention concerns controlling temperatures of electro-optical components such as attenuators, filters and solid state lasers. For use in optical communication. Moreover, although not exclusively, the invention concerns an optical attenuator with an enhanced resolution for use in an optical communication system.
It is conventional practice to employ optical attenuators in optical communication systems for regulating and controlling the power of optical radiation propagating within the systems. Such attenuation is necessary in order to avoid saturating sensitive optical components such as detectors and optical amplifiers, as well as ensuring that optical radiation is of sufficient power not to be swamped by noise. Saturation can lead to loss of information and hence errors in communication traffic conveyed by the systems.
Conventional optical attenuators employ a number of different optical component configurations, for example they can comprise one or more of Mach-Zehnder interferometers, modulated liquid crystal shutters and dispersion effect modulators. In communication systems, it is particularly convenient to employ thermally variable optical attenuators whose optical attenuation is determined by attenuator temperature. Thus, attenuation can be selected in these thermally variable attenuators by adjusting their temperature.
Temperature adjustment is conveniently achieved by including thermoelectric elements into the variable attenuators. Such elements function by the Seebeck effect and can selectively cool or heat attenuation determining optical components incorporated within the attenuators. However, the elements often consume significant power in operation, for example 2.5 Watts corresponding to an electrical drive signal of 5 volts potential at 0.5 amps current.
Conventional optical communication systems are typically configured as a plurality of nodes interconnected by optical fibre waveguides through which communication traffic bearing. optical radiation propagates from one node to another. The nodes often comprise a considerable array of optical and electrical signal processing equipment usually arranged into equipment racks, for example conventional 19-inch racks. The equipment typically incorporates numerous examples of the aforementioned thermally variable attenuator. On account of inclusion of such examples, thermal power dissipation from the attenuators can represent a considerable thermal load in the equipment racks requiring cooling facilities, for example fans for providing cooling airflow through the racks.
The inventors have appreciated that, whereas it is not feasible to reduce thermal dissipation within the attenuators because such dissipation is dictated by fundamental characteristics of their associated thermoelectric elements, it is beneficial to reduce power dissipation within electrical driver circuits which provide power to the attenuators. It is known practice when driving thermoelectric elements to regulate the drive current using a conventional circuit comprising linear non-switching components such as series regulating bipolar power transistors driven by conventional analogue operational amplifiers. Such a circuit suffers a drawback that power dissipation within the power transistors can approach power dissipation occurring within their associated thermoelectric element. In order to address this drawback, the inventors have devised a circuit for driving a thermoelectric element of a thermally variable optical attenuator wherein the circuit employs pulse width modulation (PWM) techniques for generating a drive signal for driving the thermoelectric element, the circuit exhibiting reduced power dissipation compared to the aforementioned conventional circuit. However, the inventors have found that such PWM techniques provide insufficient resolution of attenuator temperature control when the drive signal is synthesised digitally in a known manner. Such insufficient resolution gives rise to corresponding lack of resolution of optical attenuation which creates problems in associated communication systems.
As is known a conventional PWM signal comprises a stream of repetitive pulses, each pulse having a duration tp and separated from neighbouring pulses thereto by a null period of duration tn. Thus, the pulses are repeated at a period of tp+tn and an average value V of the PWM signal is given by Equation 1 (Eq. 1):                     V        =                                                            (                                  A                  -                  B                                )                            ·                              t                p                                                    (                                                t                  n                                +                                  t                  p                                            )                                +          B                                    Eq        .                  xe2x80x83                ⁢        1            
where
A=signal value during the pulses; and
B=signal value during the null period.
Moreover, the pulses have a repetition frequency fp determined by Equation 2 (Eq. 2):                               f          p                =                  1                      (                                          t                n                            +                              t                p                                      )                                              Eq        .                  xe2x80x83                ⁢        2            
In contemporary pulse width modulator design, pulses are often generated by digital counter circuits operating at a clocking frequency fclk of a master clock. As a consequence of employing such digital circuits, the durations tp and tn can only be modified in discrete steps, the number of steps M being determinable from Equation 3 (Eq. 3):                     M        =                              f            clk                                f            p                                              Eq        .                  xe2x80x83                ⁢        3            
In order to increase the number of steps M, either fclk must be increased or a lower pulse repetition frequency fp must be accepted. In some applications, temporal variations in the value V can cause problems and thereby sets a lower limit on fp. A conventional approach when enhanced PWM resolution is desired is to use a higher master clock frequency fclk; such an approach results in greater cost associated with the digital circuits and also greater operating power dissipation therein. Moreover, there are practical limits to the frequency at which digital circuits can be clocked.
The inventors have appreciated that the number of steps M can effectively be increased by grouping the pulses into frames of F pulses where one or more pulses of each frame are made to have one step greater time duration than other pulses in the frame. Such a pulse frame technique increases the number of resolution steps to a value MF and results in a total temporal fluctuation not exceeding one step. Moreover, the inventors have appreciated that there are particular approaches to selecting the one or more pulses to be made one step greater which result in a relatively low harmonic content in a PWM signal thereby generated. Reducing harmonic content is important where the PWM signal is used to control appreciable current, for example, within an optical communication system where it is important to suppress interference between electronic assemblies resulting from PWM current surges.
The present invention has arisen in an endeavour to provide an optical attenuator and associated control circuit which provide power efficiency attributable to PWM operation but a variable attenuation of sufficient resolution for use in optical communication systems.
According to a first aspect of the invention there is provided a method of generating a pulse width modulated (PWM) signal in response to a digital demand data word which comprises a plurality of bits, the method characterised by: generating the PWM signal comprising a sequence of frames, each frame comprising a train of PWM pulses whose duty cycle is substantially governed by a plurality of the more significant bits of the demand data and in response to each of a plurality of the less significant bit of the demand data, modifying at least one of the PWM pulses, wherein the number of PWM pulses being modified and their position within the frame are selected such as to uniquely map each less significant bit onto associated PWM pulses.
The method provides the advantage that it is capable of providing a PWM signal having a reduced harmonic content in comparison to conventional methods of generating PWM signals.
Preferably the method comprises generating the PWM signal from a clock signal and is further characterised by modifying the duty cycle of the PWM pulses by an amount corresponding to a single clock cycle.
In a preferred implementation the PWM pulses to be modified in response to each less significant bit, biti where i is an index in the range 0 to Qxe2x88x921 and Q is the number of less significant bits, are selected using the relationship:
xe2x80x83Sp=INT(2Qxe2x88x92i)
where INT corresponds to an integer function and Sp is the spacing of pulses to be modified. Such a spacing of the modified pulses is capable of ensuring that selected pulses for each bit are uniformly distributed throughout each frame, thereby reducing harmonic content of the PWM signal.
Preferably the first pulse Cp in each frame to have its duty cycle modified in response to a less significant biti, for values of i=1 to Qxe2x88x921, is defined by:
Cp=xc2xdINT(2Qxe2x88x92i).
Selecting the first pulse in such a manner again ensures that selected pulses are uniformly distributed from frame to frame for a given bit set. Moreover, in the special case of the least significant bit of the demand data, i.e. bit0 where i=0, the first pulse in each frame to be modified is given by Cp=Sp. Such a selection is capable of simplifying digital circuit design.
In one example embodiment of the invention, it is convenient that each digital demand word comprises 15 bits of which there are 8 more significant bits and 7 less significant bits.
According to a further aspect of the present invention, there is provided apparatus for generating the PWM signal according to the above method. Preferably the apparatus includes clock means for generating a clock signal for processing according to the method to generate a corresponding PWM signal.
Preferably, the apparatus further comprises switching means for supplying electrical current to a load in response to the PWM signal. Advantageously the switching means are coupled in a bridge configuration. The apparatus provides the benefit of being capable of presenting the load with a PWM signal including fewer harmonic components in comparison to conventional PWM apparatus.
Conveniently, it is desirable to incorporate filtering means for attenuating relatively higher harmonic components of the PWM signal from reaching the load. Such filtration results in the load experiencing less harmonic components which can temporally influence its operation. Preferably, the filtering means comprises passive components having an impedance which is substantially reactive such as for example at least one inductor and at least one capacitor. More preferably, for reasons of physical compactness and potentially lower cost, the at least one inductor is ferrite cored.
Preferably, the load comprises a thermoelectric element. Such thermoelectric elements are known to require relatively high currents to operate, pulsed switching of these currents being a source of interference in electronic systems. Thus, the apparatus according to the invention is especially appropriate for use in controlling power to such an element.
Preferably, the element is thermally coupled to one or more of an optical attenuator, a laser and an optical filter for controlling their temperature and thereby their optical characteristics. The apparatus is capable of providing less electrical interference to the operation of such an attenuator, a laser and a filter in comparison to conventional PWM controllers.
The present invention finds particular application for controlling temperatures and thereby attenuation of optical attenuators for use in an optical communication system. Thus according to a further aspect of the invention there is provided an optical attenuator for receiving input radiation and attenuating the input radiation in response to a drive signal to provide corresponding output radiation, said attenuating means being operable to provide an attenuation dependent upon its temperature; the attenuator including temperature modifying means for modifying its temperature in response to the drive signal and controlling means for receiving a signal indicative of attenuation required and for generating the corresponding drive signal characterised in that the drive signal comprises a PWM signal in which each PWM signal cycle corresponds to a frame, a plurality of such successive forming a multiframe and in which the controlling means is operable to modify the duty ratio of one or more frames within each multiframe for providing enhanced resolution attenuation provided by the attenuator.
The invention provides the advantage that the attenuator is capable of providing an enhanced degree of attenuation resolution together with power efficiency benefits associated with using the PWM drive signals in accordance with the present invention.
Conveniently, the controlling means includes filtering means for filtering the PWM signal to generate the drive signal. Such filtration provides a benefit that PWM cycle fluctuations are not directly experienced by the attenuating means and therefore less likely to be transmitted onto the output radiation.
The PWM signal is preferably of substantially constant cycle period, and the filtering means is operable to attenuate PWM signal components at a frequency corresponding to the cycle period. Such filtration is effective at removing principal fluctuating harmonic signal components present in the PWM signal. Conveniently, the filter is a passive filter comprising inductors and capacitors; such inductors and capacitors have low energy losses associated therewith and hence provide energy efficient conversion of the PWM signal to generate the drive signal.
Advantageously, modifications to the duty ratio of frames within each multiframe are substantially uniformly distributed within the multiframe. Such uniform distribution assists to reduce the magnitude of relatively low frequency perturbations in the drive signal. The duty ratio of each frame is preferably incrementable in discrete steps, the modifications to the duty ratio of the frames corresponding to one such step difference.
Conveniently, each multiframe comprises in a range of 2 to 1000 frames. This range provides a compromise between enhanced resolution and lower frequency signal fluctuations present in the drive signal. Preferably, each multiframe comprises 64 frames as an optimal compromise.
Advantageously, the attenuator is capable of providing radiation power stabilisation of the output radiation by using a negative feedback loop. In order to provide such stabilisation, the attenuator further comprises: detecting means for receiving a portion of the output radiation and generating a corresponding detection signal; amplifying means within the controlling means for comparing the detection signal with a reference signal and for adjusting via the temperature modifying means the temperature of the attenuating means so that the output radiation has associated therewith a radiation power determined by the reference signal.
When implementing the attenuator in practice, it is desirable the controlling means is implemented as a field programmable gate array (FPGA). Use of the FPGA provides benefits of reconfigurability whilst employing relatively few electronic components. Preferably, the FPGA is clocked at a rate of at least 30 MHz.
The temperature modifying means requires appreciable current to operate when providing a relatively high degree of heating or cooling of the attenuating means. Thus, conveniently, the FPGA is operable to generate a PWM signal which is buffered by power MOSFETs for output to drive the attenuating means.